Coding and multiple-access over fading channels

Knopp, Raymond

Thesis

The field of wireless radio communications is undoubtedly one of the most active and economically rewarding sectors in technology today. Existing terrestrial cellular networks already offer both voice and data services at reasonably affordable prices and there will soon be satellite networks which will offer communication services to and from any point on the globe. This thesis takes a fundamental look at the communication problem over so-called fading channels which are the types of channels encountered in many radio communication systems. The main obstacle that the radio system designer has to cope with is the channel's underlying time-varying and time-dispersive nature. We strive towards a better understanding of the fundamental limits for the communication process over such channels and at the same time, wherever possible, indicate ways for approaching these limits with practical devices. Moreover, in many cases we use channel models which accurately describe the physical media, at the expense of giving up the possibility of presenting analytical solutions. We show that the channel is prone to outages, in the sense that there is irreducible probability that reliable communication is impossible. These outages can only be avoided if there is some form of channel state feedback from the receiver to the transmitter. We discuss issues such as coding and power control and how they can be used jointly to improve performance both for long-term and short-term measures. Spread-spectrum systems are treated in a general sense and different coding alternatives are compared for such applications. We examine coding schemes for a particular class of fading channels, known as block-fading channels and show that very practical codes can come close to fundamental limits on performance. Moreover, we have shown that there is a bound on the fundamental performance of such codes which depends on several design factors. We have found a series of block and trellis codes for moderate spectral efficiencies and present computer simulation of their performance. The last part of this work is concerned with the multiple-access problem over such channels, which is the problem of sharing a common radio medium between a collection of user terminals wishing to communicate with a single base-station. We show that by performing a certain type of dynamic channel allocation using channel state information at the user terminals, we can achieve performances which surpass those of a non-fading environment. The development is simple and relies on the time-varying nature of the fading channel.